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Abstract

A Little Tikes Patrol Police Car was retrofitted with a battery-powered emergency police lights and siren system as an entry to the Dangerous Prototypes Open 7400 Logic Competition. The circuit design focuses on simplicity and efficiency of part utilization by using only two 7400 series integrated circuits. A 5-stage Johnson decade counter is used to modulate the frequency of the warbling siren and to drive two rotating beacons consisting of ten LEDs each. A hex inverter is used to rotate the beacons, drive the four strobe LEDs, and drive the siren's primary carrier tone. This report discusses the design, testing, modifications, construction, and reception of the project.

Introduction

My children have had a Little Tikes Police Car for some time now. The faux emergency light bar only has a reflective material under the red and blue lenses, but no lights. It has always seemed odd to me that a child's police car with a light bar wouldn't have lights in it. This needed to be fixed.

No lights in a kids police car that has a light bar? Are you kidding?!?

Concept

I've always enjoyed tinkering with electronics, but unfortunately the tinkering is pretty low on my priority list. The Dangerous Prototypes Open 7400 Logic Competition offered an opportunity and excuse to make a project and actually complete it. Previous unfinished projects were a bit too ambitious for my schedule, so I figured a simpler circuit that focused on being minimalistic would have a better chance of being completed. The emergency police light for the Little Tikes car seemed like the right fit for this competition.

The solution first came to me over a year ago when I saw a back-and-forth Cylon/Knight Rider light that used an oscillator and a 4017 Johnson counter. The following schematic from http://http://www.evilmadscientist.com comes from an example circuit.

For the lights that are powered by two channels of the counter, each counter channel has a diode in series to allow independent driving of the same LED, while not over-driving the counter-channels by shorting them together. What if, instead, one were to use a single counter channel for each LED? You'd then have a string of lights. Then, arranging them into a circular pattern one could create a classical rotating police beacon lighting effect. This seemed like a good solution for the rotating police light.

Last edited by nato on Sat Oct 22, 2011 7:18 am, edited 6 times in total.

The prototype circuit was first constructed on a solderless breadboard.

Completed prototype

The resistor values were chosen so that the maximum output currents of the ICs specified in their respective datasheets would not be exceeded. Exceeding these limits is not recommended by the manufacturer as doing so could damage the components. For other portions of the circuit, the resistor and capacitor values were chosen so that the right frequencies were created for realistic effects. These decisions are detailed further on. The following is the final version of the circuit:

I dedicate this hand-drawn schematic to Forrest Mims; reference this schematic when reading the following discussion.

I decided that the power supply had to be portable and fit within the center portion of the light bar. It had to supply enough current to light multiple bright LEDs and a siren simultaneously, so watch-size batteries were out. For efficiency sake, AAAs were chosen over a 9 volt battery as using a 9-volt would result in most of the energy being dissipated as heat when powering the lower voltage LEDs. 3 AAAs were chosen as their nominal voltage when configured in series were sufficiently above the maximum forward voltage of any of the LEDs which would allow for operation through the entire life of the batteries.

The HC series of ICs can operate from 2.0V to 6.0V, so this series was chosen as it can operate over a wide range of battery health. Being a logic competition, it made sense to use an inverter-based oscillator to drive the counter. Two inverters of a 74HC04 hex inverter were used for this rotation driver oscillator. R1 and C1 determine the frequency of the oscillator and is used to clock the counter transitions from one channel to the next. This translates into the speed of the rotation effect. As with any oscillator that incorporates an RC network, its frequency is inversely proportional to both the resistor value and the capacitor value. I chose a .1uF capacitor as it was within the ballpark of what I needed for the correct rotation speed and because it is a common value. I then experimented with different resistor values until I found the value that produced a rotation speed that seemed realistic to me.

The frequency of the oscillator is inversely proportional to the product of the resistance and capacitance values.

06_R1-C1_Frequency.PNG (6.17 KiB) Viewed 14366 times

Connecting LEDs to outputs of a circuit that don't have any current limiting isn't wise, and I chose the simplest solution--current-limiting resistors. The data sheet for the 74HC4017 lists the maximum current output as 25mA, so my target was just below that value. As I used a 5.0V supply for prototyping and was planning on using three AAA batteries for the final project, I used 5.0V for the supply voltage when making calculations to determining maximum currents. With a higher 2.89V forward voltage of the blue LEDs when compared to the red LEDs 1.93V forward voltage, the blue LED would require less resistance to limit the current to equal that of a red LED. However, the human eye is more sensitive to the wavelength of the blue LEDs, and I found that the apparent brightnesses of the two LEDs were fairly close when I used 220 ohms for both the red and the blue LEDs. Note from the following calculations that the maximum output current is not exceeded with these components.

While the original car had a microphone with a siren capability, the microphone was lost before we acquired this hand-me-down toy from a neighbor. Since I had four additional inverters available in the 74HC04 hex inverter package, it made sense to create a siren using them. In my original design, a pair of inverters were used to construct an astable multivibrator that creates the high-pitched carrier tone of the siren, and another pair of inverters were used to construct the slow oscillator that is used to modulate the frequency of the astable multivibrator, creating the "wee-oo-wee-oo-wee-oo" warble.

The siren circuit's astable multivibrator, built with the two inverters IC1C and IC1D, drives the speaker with the high-pitched carrier tone whose frequency is determined by the R5-C3 and R6-C4 networks. Each of the networks determines one half of the square waveform period. For simplicity sake, I chose .1uF capacitors and decided to use the same value resistor for R5 and R6. Here again, the higher the resistance, the lower the frequency, and vice versa. I settled on a value that created a siren that centered around what seemed to be a realistic frequency.

The frequency of the astable multivibrator was originally modulated with an inverter-based oscillator with an identical structure as the rotation driver oscillator. This oscillator had a frequency that was close to that of the counter's carry output and was subsequently replaced with the carry output. This replacement is detailed further in this report.

Changes of the R4-C2 network component values result in a different effect than changes of the values of the other RC networks in this circuit. This RC network smooths the signal that is used to modulate the frequency of the astable multivibrator while the other RC networks control the actual frequency of their signals. The rate of change of the frequency sweep increases as the product of the resistance and capacitance values is decreased. Decreasing the RC constant also has the secondary effect of widening the frequency difference between the lowest and highest frequencies heard in the siren sweeps.

Decreasing the R4-C2 constant increases the slope of the frequency curves and the frequency limits, but does not change the period of the sweeps. In the final circuit, the period of the sweeps is determined by the frequency of the counter's carry output.

When first testing the siren, the siren started at an inappropriately high frequency when first powering the device. It would then take a few seconds for the siren frequency to work its way down to a natural pattern that sounded correct. I significantly improved this by lowering the value of C2. This resulted in the capacitor charging more quickly so that the modulator reached equilibrium in less than a second.

The speaker I had on hand came from a busted set of headphones. As the speaker was only 32 ohms, I had to limit the current with a 220 ohm series resistor to prevent over-driving the inverter. This had the effect of significantly reducing the volume that I may have obtained had I had a speaker with an impedance closer to the current-limiting resistance. However, after a couple hours of the children playing cops, I'm glad I had to make this "compromise" in component selection.

Near the end of the prototyping stage, I returned my focus again to simplicity. I first eliminated the series resistors that I had with each of the red and blue LEDs. As only one of the LEDs in each ring would be powered at any one time, it became obvious that I only needed one resistor per LED ring. This netted me a savings of 18 passive components.

Also, I discovered that the Johnson counter had an unused carry output that the referenced datasheet calls the "most significant flip-flop". This output, as configured, has an oscillation frequency similar to that of the siren's frequency modulator oscillator. I decided to use the carry output in place of the previously used frequency modulator oscillator. This freed up two of the inverters.

Modern emergency light arrays usually incorporate some strobe lights. I used the newly freed inverters to create the strobe light effects. The white LEDs used for the strobe light effects have a forward voltage of about 2.91V.

As I wanted a harsh bright flash effect for the strobes, I exceeded the 25mA continuous current rating of the inverters by 3mA, or about 10%. I justify doing this because it isn't a commercial product, there are no safety issues involved, and the duty cycle for these lights is only 20%.

The inverters that power the white LEDs are controlled by outputs from the counter fed through diodes. Like the Cylon circuit discussed above, these diodes allow the counter channels to independently control the same inverter, while not over-driving the counter channels by shorting them together.

Considering this was going to be used with three AAA cells, I checked the operation of the circuit over a wide range of voltages. The circuit worked fine from 5.0V down to 2.8V at which point the brightness of the white LEDs were very low. The circuit seemed to lose the harsh brightness one would expect from a strobe light when the supply voltage dropped below about 3.3V. With these voltages, an average of 1.1V to 1.67V is required per cell to offer a good show. This fits well with my desire to completely deplete the batteries before having to replace them.

Last edited by nato on Fri Oct 21, 2011 9:41 pm, edited 3 times in total.

I decided to construct the circuit on a perfboard. All of the board-mounted components would be placed on a perfboard with one of the LED beacon rings, and the other LED beacon ring would be placed on a perfboard by itself. Attention was paid to the orientation of the LED beacon rings--they were set to rotate in opposite directions, and offset so that no more than one ring would be lit on the front or rear direction at any particular time. Additionally, I intended that no strobe lights would be activated the instant that either of the ring's position were at the immediate front or immediate rear position. The following layouts of the perfboards aim to fulfill these criteria and to minimize the number of jumpers used.

Component layout of primary perfboard

Component layout of red LED beacon ring perfboard

Shortly after beginning the soldering of the circuit, I determined that my barely-used Aoyue 937+ soldering station was not working correctly. The temperature would not ramp up nearly as quickly as it normally did, and it would take a very long time to heat the components and solder. After wasting a couple hours with this I finally switched to using my trusty Weller SP23 fire-starter with which I performed 90% of the work of this project.

I soldered a few components at a time, testing the circuit at different points to avoid getting too far along with a mistake. This made debugging the circuit easier. I first connected the ICs and the Gnd and VCC buses and constructed the rotation driver oscillator and connected a single blue LED. After ensuring that this worked, I continued adding a few components at a time and testing that they produced the expected results.

The layout originally had a few surface mount components. These were used in the siren portion where they could easily save space and jumpers. Unfortunately, after the siren portion of the circuit was constructed, the siren only emitted a clicking sound. I performed the following steps, testing the circuit at each step to try to find the cause:

I double-checked and triple-checked the perfboard circuit to the layout and then to the schematic.

I examined the perfboard circuit under a loupe looking for shorts and disconnects.

I also checked and double-checked the circuit with the multimeter looking for shorts and opens where they should and shouldn't be, checking that the resistances were correct and that the capacitances were correct.

Thinking that I may have fried or semi-fried the inverter IC, I replaced it.

Then, thinking that my schematic must not have represented the actual prototyped breadboard circuit, I reconstructed the siren portion of my circuit on the breadboard based on the schematic. This breadboarded circuit worked.

I wasted a *lot* of time trying to figure out why the siren wasn't working, and nearly gave up on the entire project. I finally decided I needed to just give up on the existing physical construction of the siren. I removed the components and connections from the siren portion of the circuit, designed a different layout using through-hole components and rebuilt the siren portion of the circuit on the perfboard. This new construction worked correctly.

The LEDs of the beacon rings were soldered with the LED lenses about 3/4" (2 cm) above the perfboard so the light would not be obscured by the other components on the board. Also, these LEDs were bent 90 degrees so that they pointed outward. This resulted in the light being projected outward in a circular pattern, and increased the realism of the rotation.

After the perfboards were finished, the light bar fixture was removed from the roof of the car. The reflective material portion of the fixture was cut away with a Dremel tool to allow installation of the circuit boards. Holes for the strobe lights were drilled into the front and rear corners of the gray housing and the strobe lights were super-glued in the holes, and held in place with binder clips while the glue set. The actual LEDs used for the strobe lights were of the flat-top type allowing much greater dispersion of the light when compared to the common T1-3/4 type 5mm LED lens of the LEDs used for the beacons that direct most of their light within a 30 degree cone.

About ready to put it all together

To help keep the wires from being tangled, I used a couple of nylon zip ties.

A piece of plywood was cut into a shape that would hold the perfboards from shifting around after the light bar fixture was screwed back into place. A small hole for the switch was drilled in the ceiling of the car's passenger compartment, and a larger hole was drilled in the top of the roof to allow soldering of the switch. This hole was covered by the light bar fixture after it was reinstalled.

After screwing back together, plywood is just the right thickness to hold the circuit boards in place.

Last edited by nato on Fri Oct 21, 2011 10:03 pm, edited 2 times in total.

Admittedly, modern police cars do not use rotating beacons, but the combined effect of a pair of rotating beacons and flashing strobes looks cool and seems believable. A scientific poll was taken to measure the objective awesomeness factor of the police light and siren system. The results can be seen below.

Objective measurements using scientific, peer-reviewed processes.

As can be seen, the system tested extremely well in all demographics except adult females. The reaction by one unnamed adult female test subject consisted of a rolling of the eyes and the comment of "that's what you've been working on all of this time?!?". I additionally note that the system may pose a health risk to toddler males as the cries of reaction by a 3-year old male subject seemed to indicate intense physical pain when being told that he had to stop playing with the police car and go to bed.

Future Developments

If time permits I would really like to transfer the schematic into a CAD program and release practical PCB artwork. I envision a simple single-sided through-hole board that could be produced at home, and would include holes that would provide the option of separating the rotating beacon ring portions of the PCB and attaching them with a cable. I believe this could be useful for educational purposes to teach some of the principles discussed in this report, to learn soldering skills, and to learn about making PCBs.

Conclusion

The omission of lights from Little Tikes Patrol Police Car seems like an obvious oversight. This situation has been remedied for at least one of their cars now. I had a fun time designing, constructing, debugging and modifying the police lights and sirens and seeing my kids' reaction to it. I hope that somebody can benefit from the included discussion of how I chose the values for the passive component as I don't think that's something that is explained very often in these types of write-ups.

As the presented information is merely a representation of my ideas, I admit that I have no exclusive right to the information or any right to your use of it. If you want to be nice, however, you'll give me credit if you use a substantial portion.